1. Field of the Invention
The present invention relates to a magnetic thin-film element using a magnetoresistance effect such as a spin tunneling effect and an application of this element and, more particularly, to an application of a nonvolatile memory.
2. Related Background Art
A magnetic thin-film memory is a solid-state memory having no movable portion, like a semiconductor memory. The magnetic thin-film memory is more advantageous than the semiconductor memory because the magnetic thin-film memory keeps information even upon power-off, has an infinite repetitive rewrite count, and is free from the risk of losing recording contents even by incident radiation. Recently, a thin-film magnetic memory using a spin tunneling (TMR) effect has received a great deal of attention because it can obtain a larger output than a conventional magnetic thin-film memory using an anisotropic magnetoresistance effect. These magnetic memories are called MRAMs (Magnetic Random Access Memories) due to their random access characteristics.
As the memory cell area decrease, the magnetic thin-film memory with the above arrangement cannot ignore a demagnetizing field (self-demagnetizing field) generated in a magnetic layer. The magnetization direction of a recording/sustaining magnetic layer cannot settle and becomes unstable. Hence, the magnetic thin-film memory having the above arrangement cannot save information at the same time as miniaturization of a bit cell, and cannot be integrated at a high degree.
To solve the integration problem of the longitudinal magnetization film, the present inventor discloses a magnetoresistive element using a perpendicular magnetization film in U.S. Pat. No. 6,219,275. This magnetoresistive element has a superior characteristic of stably conserving magnetization even upon miniaturization.
The longitudinal magnetization film is known to increase its inversion field along with miniaturization. For example, it is reported in Journal of Applied Physics (J. Appl. Phys.) Vol. 81, p. 3992, 1997 that the inversion field increases in proportion to the reciprocal of the width in the longitudinal magnetization film and reaches as large as about 260 Oe for 0.25 μm. If the magnetic film is miniaturized, the inversion field is considered to further increase.
In the MRAM, information is recorded by the switching magnetization of a magnetic film by a magnetic field generated by supplying a current to a write line. An increase in inversion field increases the write current and power consumption. Furthermore, recording itself fails if a current equal to or higher than the limiting current density is required.
An increase in inversion field along with miniaturization may also occur in a perpendicular magnetization film. Miniaturization with a constant film thickness increases the shape anisotropy (to be described later). The magnetization occurs more readily in the perpendicular direction, thus increasing energy accompanying switching.
The present invention has been made in consideration of the above situation, and has as its object to provide a magnetic thin-film element which exhibits a magnetoresistance effect and is thermally stable in an MRAM using a perpendicular magnetization film without any increase in inversion field which poses a problem in miniaturization.